fix: remember downloaded cloud snippets after reload

[imantsk]

Restores the download state of community cloud snippets across a page load, and matches the two Playwright tests that have failed on core-beta since 74aba6f8 to the plugin's actual behaviour.

Downloaded cloud snippets forget themselves on reload

Downloading a snippet from the Community Cloud stores the remote identifier on the local snippet, and the card switches from Download to Edit. Reloading the page returned the card to Download: the search and featured responses never carried a local_id, so nothing but the in-memory download record knew the snippet was already on the site.

The REST controller now maps each cloud snippet to a local snippet through the cloud_id recorded at download time, for both search and featured results. Trashed snippets are excluded. Cloud_Snippet::local_id keeps a null value instead of coercing it to 0, so a snippet that was never downloaded is reported as such.

Back navigation confirmation

The test asserted that window.history.back() raises a beforeunload dialog. Whether the browser raises beforeunload or the editor's own popstate handler raises window.confirm depends on how the editor was reached, and both occur in practice, so the assertion failed in either direction depending on the run. The test now answers whichever prompt appears and asserts the outcome: dismissing keeps the editor open with the edit intact, accepting leaves it.

Shared cloud download state

The test froze the search refresh that follows a download and failed it with a 500, then asserted that the preview still offered editing. A failed search clears the results, unmounting the cards and the preview rendered inside them, so the assertion ran against an empty page.

The refresh is now held open briefly instead of being failed, and the assertions run once the download response has been received rather than while it is resolving. The card is checked before the refresh returns and again after it reports the snippet as still not downloaded. The error notice assertions were dropped as the search-failure test above already covers them.

Assertions on the preview after the download were dropped as well. The preview is a @wordpress/components modal supplied by WordPress rather than bundled with the plugin, and replacing the focused download button with a link closes it on some WordPress versions while leaving it open on others. It stays open on 7.0.2 and closes on the version used by the Playwright workflow. Keeping focus on the snippet as the button changes would settle this, and is worth its own change.

Verification

• PHPUnit: 144 tests, including new coverage for local IDs in search and featured responses.
• Playwright: the Community Cloud Featured suite and both back navigation tests pass, run repeatedly to confirm they are stable.
• Checked by hand against a site with 91 locally stored cloud snippets: cards for snippets held locally offer editing after a reload, and cards for snippets that are not held locally still offer Download.